The present invention relates to a steering angle (steering-wheel turning angle) detecting apparatus for use in motor vehicles.
Generally, for detecting a steering angle of a motor vehicle due to operation of a steering wheel, a rotary encoder is attached to a steering shaft so as to output two-phase pulse signals which are in turn up-and-down-counted and sampled to calculate the absolute value of the steering angle. The calculation of the absolute value of the steering angle can be determined on the basis of the count value of the two-phase pulse signals when the motor vehicle is straightforwardly running, i.e., when the steering wheel is taking the neutral position. However, one of problems with such a steering angle detecting arrangement is that it is difficult to detect the neutral position of the steering wheel because the steering wheel allows about three revolutions. In addition, the neutral position of the steering wheel must be detected at least one time after the turning-on of a keyswitch of the motor vehicle. This is because the neutral position (or count value) can be shifted when the keyswitch is turned on due to the movement of the steering wheel after the keyswitch had been turned off. One possible solution for this problem is to detect the neutral position on the basis of the average value of the steering angle signals obtained when the steering neutral zone signal is detected such as disclosed in the Japanese Patent Provisional Publication No. 61-28811. Another possible solution is to correct and update the neutral position by increasingly changing the weight of the steering angle position as the motor vehicle more takes the straight-running state. There are problems which arise with such techniques, however, in that the neutral position can be obtained only in a limited steering angle range and difficulty is encountered to accurately obtain the steering angle neutral position when the motor vehicle is out of this limited range (a.k.a. turning).